1. Field of the Invention
The present invention generally relates to a digital transmission system, and more particularly, to a digital transmission device such as a facsimile machine having an error correction mode and a method for shifting down a data transmission rate at a transmitter in response to a request from a receiver for retransmission of data frames having data errors in a digital transmission system system.
2. Discussion of the Background
In a facsimile transmission system, a receiver will not receive correct image information when a transmission error (also referred to as a data error) occurs due to the poor condition of the transmission line such as a wire line or a radio line being used. Presently, in order to cope with the occurrence of transmission errors, facsimile machines have error correction functions (modes), which are different for different manufacturers. Such facsimile machines can achieve image information transmission without error.
However, in many cases, there is no interchangeability among the error correction modes of different manufacturers. That is, a facsimile machine having one type of error correction mode cannot communicate with another facsimile machine having a different type error correction mode, when a transmission error occurs.
From the above-mentioned viewpoint, a standard procedure relating to error correction modes in facsimile transmission has been proposed by Comite Consultatif International Telegraphique et Telephonique (hereafter simply referred to CCITT). The proposed standard procedure is issued as Annex A of the recommendation T.30 which defines functions of a group III facsimile machine. According to the recommendation T.30, image information after coding and compressing image data is divided into sub-frames each having 256 bytes (1 byte amounts to 8 bits) or 64 bytes. Then, as shown in FIG. 1A, image information amounting to one frame is arranged into a frame FLM defined by a high-level data link control (HDLC) frame format. Each frame FLM consists of a (beginning) flag sequence F consisting of a predetermined bit pattern, an address field A consisting of a predetermined bit pattern (global address), a control field C consisting of a bit pattern inherent in the type of facsimile machines, an information field I, a frame check sequence FCS, and a (end) flag F. These contents of the HDLC frame formats are arranged in the above-mentioned order from the beginning of the frame FLM.
The information field I consists of a facsimile control field FCF in which a facsimile transmission procedure signal is defined, and a facsimile information field FIF in which there is defined a variety of information to be added to the facsimile transmission procedure signal. In the facsimile control field FCF, there is disposed a facsimile coded data FCD of the facsimile transmission procedure signal. In the facsimile information field FIF, there are arranged a frame number FNo which represents the sequence of frames, and coded frame data FDc of one frame size FSZ. Since the frame number FNo consists of binary digits consisting of 8 bits, the frame number FNo is capable of representing consecutive numbers of only 0-255. For this reason, one block is defined with 256 consecutive frames, and a request for retransmission is generated for every block. In a case where image information amounting to one page cannot be transmitted with one block, the remaining data is transmitted so as to be set in the next block and transmitted.
When a request for retransmission is generated at a receiver, the receiver sends the transmitter a data frame having a facsimile transmission procedure signal PPR (a partial page request signal) as shown in FIG. 1 In the following description, the data frame having a facsimile transmission procedure signal PPR is simply referred to as a procedure signal PPR. A variety of data frames having related facsimile transmission procedure signals is actually used. For convenience' sake, these data frames are simply referred to as procedure signals.
The procedure signal PPR includes a bit pattern which indicates that the present signal is the procedure signal PPR and which is arranged in the facsimile control field FCF, and error map data EMp consisting of 256 bits which are arranged in the facsimile information field FIF. Each of the error map data EMp indicates the presence/non-presence of a transmission error for the respective frame data amounting to one block. Each error map data EMp has data `0` given a frame in which there occurs no transmission error, and data `1` given a frame in which a retransmission error occurs. The error map data EMp are arranged in the order of frames. When receiving the procedure signal PPR, the transmitter retransmits frame data relating to the frames to which the error map data EMp are set equal to "1". The request for retransmission is repetitively generated until all the frames have no transmission error. Thereby, the receiver can obtain the correct image information sent from the transmitter.
When the retransmission request for the same block is repetitively generated many times, it takes longer to transmit data. Therefore, when the retransmission request is repetitively generated a predetermined number of times, the transmission rate is shifted down by one step. The above is proposed by the aforementioned CCITT recommendation. This is described below with reference to FIG. 2.
When phase C indicating the transmission of image information is activated, image information equal to one block is transmitted (step 101). Subsequently, a signal PPS-Q (Q denotes is MPS, EOM, EOP, PRI-Q) that instructs a related operation to be executed after transmitting image information is sent to the receiver (step 102). Thereafter, the transmitter receives a response from the receiver.
The transmitter determines whether or not the transmission procedure signal indicated by the received response is the procedure signal PPR (step 104). When the result in step 104 is YES, the transmitter determines whether or not the received procedure signal PPR has been received for the M-th time. (step 105). When the result in step 105 is NO, the transmitter forms retransmission image information consisting of frames each having a transmission error (error frames) informed by the procedure signal PPR (step 106). Then the procedure returns to step 101.
On the other hand, the result in step 105 is YES when the number of times that the retransmission of the same block is requested, reaches a predetermined number. Thus, the transmitter sends the receiver a continue-to-correct signal CTC used for informing the receiver that the transmission rate is shifted down by one step (step 107). Then the transmitter waits for a response for continue to correct CTR, which is sent back from the receiver when it is ready for receiving image information (step 108).
When receiving the procedure signal CTR the transmitter shifts down, by one step, the transmission rate at which image information is to be sent, and forms retransmission image information in step 106. Then the procedure returns to step 101, and the retransmission image information is sent to the receiver at the shifted-down transmission rate. On the other hand, when the result in step 104 is NO, the procedure is shifted to the process related to the contents of the procedure signal which was received in step 103. Then phase C ends.
In this manner, when the number of times that the retransmission of the same block is requested, reaches (m-1) times, the transmission rate to be set in the next transmission is shifted down by one step.
However, in the conventional retransmission procedure, the number of times that the same block is retransmitted, is always monitored irrespective of circumstances where transmission errors occur. For this reason, it takes extremely long to execute the retransmission procedure when transmission errors occur frequently due to deteriorated conditions on transmission lines, and transmission errors are not eliminated after the predetermined number of retransmission processes is repetitively carried out.
In order to eliminate the above-mentioned disadvantages, an improvement in retransmission in facsimile systems has been proposed in U.S. Pat. application Ser. No. 07/253,621 (the assignee of which is the same as that of the present application), the disclosure of which is hereby incorporated by reference. In the proposal, the transmission rate used for retransmitting image information is shifted down on the basis of the error ratio of the number of error frames informed by a receiver to the number of frames sent to the receiver. However, the shift-down process based on the error ratio, cannot effectively cope with a variety of circumstances where transmission errors occur. Therefore, there is plenty of room for improvement.